Designing MoS2 nanocatalysts with ultrasmall size and few layers is an effective strategy to enhance their electrocatalytic activity in the hydrogen evolution reaction (HER). In this work, uniform MoS2 quantum dots (QDs) are synthesized by a novel and facile hydrothermal route. Transmission electron microscopy and atomic force microscopy measurements show that the resulting products possess a monolayer thickness with an average size of about 3.6 nm. Benefiting from the abundance of exposed catalytic edge sites, as well as the excellent intrinsic conductivity of the monolayer structure, the MoS2 QDs showed excellent catalytic activity with a low overpotential of approximately 160 mV and a small Tafel slope of 59 mV dec(-1), which made it a promising HER electrocatalyst for practical applications.